There exists an ongoing need for acoustic wave filters and delay lines for a variety of practical applications. Acoustic wave devices are becoming particularly important in the production of electronic signal processing equipment, especially radios, because they can be readily mass produced and are of small size and also because of increased pressure to employ available radio spectrum efficiently. Acoustic wave devices are generally constructed on planar surfaces using techniques similar to those employed in manufacturing integrated circuits.
Acoustic wave filters employ generally periodic arrays of electrodes configured to provide discrete elements such as transducers (for converting electrical to mechanical energy and vice versa), reflectors (for reversing the direction of propagation of an acoustic wave) and gratings (e.g., for separating transducers and/or resonant cavities and/or providing electrical isolation therebetween). These elements are grouped in a generally in-line configuration (e.g., reflector, transducer, grating, transducer, reflector) and are separated by inter-element gaps, with the entire array providing an electrical filtering function associated with the electrical port(s) of the transducer(s).
The desired electrical performance often dictates that the composite filter employ more than two acoustically coupled transducers, external matching and band-shaping components or involves other (e.g. size-related) requirements not consistent with the needs of a specific application. Failure to include these elements often leads to bandwidth perturbations from the desired, and/or increased in-band (i.e., passband) insertion loss and decreased out-of-band (i.e., stop-band) rejection.
What are needed are an apparatus and a method for making acoustic wave filters having electrical performance characteristics that achieve a combination of frequency selection characteristics, insertion loss and footprint (i.e., package areal mounting requirements) not previously available for filters comprising a single pair of acoustically coupled transducers.